1. Field of the Invention
The present invention relates to an objective lens, an optical pickup apparatus, and an optical disk apparatus using the same. More specifically the present invention concerns an objective lens for focusing light beams on optical disks such as a magneto-optical disk (MO), a compact disk (CD), CD-ROM, etc. and an optical pickup apparatus and an optical disk apparatus using the same.
2. Description of the Related Art
An optical disk apparatus is widely known as an apparatus for recording information on an optical disk or reproducing information recorded in an optical disk. Generally, the optical disk apparatus comprises an optical disk drive apparatus for rotating an optical disk and an optical pickup apparatus. The optical pickup apparatus focuses on an optical disk a light beam irradiated from a light source via an objective lens controlled and driven in two axial directions, namely focusing and tracking directions, and receives return light from the optical disk.
FIG. 8 shows an example of the optical pickup apparatus. In this example, an optical pickup apparatus 1 mainly comprises a beam splitter 5, an optical detector 6, a grating element 8, and a semiconductor laser 9. The beam splitter 5 is positioned below the objective lens 3 inserted below an optical disk 2 and is slanted 45° against an optical axis 4. The optical detector 6 is arranged below the beam splitter 5. The grating element 8 and the semiconductor laser 9 are serially arranged to the side of a reflecting surface 7 of the beam splitter 5.
FIG. 9 shows an example of a drive section for driving the objective lens in two axial directions. In this example, an objective lens drive section 10 comprises a horizontally extending elastic suspension 11, an objective lens holder 12, a focusing coil 13, a tracking coil 14, the objective lens 3, a magnetic yoke 15, and a magnet 16 fixed to the yoke 15. The suspension 11 supports the objective lens holder 12, the focusing coil 13, and the tracking coil 14. The objective lens 3 is horizontally held in the objective lens holder 12.
In this configuration, the optical disk drive apparatus (not shown) rotates the optical disk 2. The objective lens drive section 10 inserts the objective lens 3 into an optical path. The light beam 17 is horizontally irradiated from the semiconductor laser 9. The grating element 8 divides the irradiated light beam into a main beam and a sub-beam. These beams are reflected upward on the reflecting surface 7 of the beam splitter 5. The reflected light beam 17 is refracted in the objective lens 3 and is focused on a signal recording surface 18 of the optical disk 2. The light beam is then reflected downward and becomes the return light beam 17. The return light beam 17 is again refracted in the objective lens 3, passes the beam splitter 5, and then enters the optical detector 6.
The optical detector 6 performs a photoelectric conversion. Based on an output detection signal, the apparatus reproduces information recorded on the signal recording surface 18 of the optical disk 2. At this time, a focusing error signal and a tracking error signal are detected. Based on these signals, a drive current for the focusing coil 13 and the tracking coil 14 is servo-controlled. A current passing through the focusing coil 13 and the tracking coil 14 interacts with a magnetic field generated by the yoke 15 and the magnet 16. Accordingly, the objective lens 3 is controlled to be driven in the focusing and tracking directions.
FIG. 10 shows the objective lens 3. The objective lens 3 comprises a convexly curved optical surface 19, another convexly curved optical surface 21, and a flange-shaped rim section 23 formed between these optical surfaces 19 and 21. A curvature radius of the optical surface 19 is relatively large. There is a length from the top of the optical surface 19 to the rim section 23 along the optical axis 4. This length is hereafter referred to as a “sag amount” and is assumed to be Z. There is a length which is optically effective for the optical surface 19 and is orthogonal to the optical axis 4. This length is hereafter referred to as an “optically effective diameter” and is assumed to be D. Under this condition, a value of Z/D generally ranges from 0.15 to 0.3.
Generally, the objective lens 3 is fabricated by using a pressing machine 24 as shown in FIG. 11. The pressing machine 24 comprises an upper mold 25 and a lower mold 26. The upper mold 25 can be lifted and lowered and is provided opposite the lower mold 26. An upper die plate 28 of the upper mold 25 is fixed to the bottom face of an upper heat insulating coupler 27. An upper die 30 made of sintered hard alloy includes an upper cavity 29 and is fixed to the bottom face of the upper die plate 28. In the upper cavity 29, there is formed a concavely curved concave section 31 which matches the shape of another optical surface 21. Like the upper mold 25, the lower mold 26 comprises a lower heat insulating coupler 32, a lower die plate 33, a lower cavity 34, and a lower die 35. In the lower cavity 34, opposite the concave section 31, there is formed a concavely curved concave section 36 which matches the shape of the optical surface 19. When the upper mold 25 is coupled with the lower mold 26, the concave sections 31 and 36 and an inner wall of the lower die 35 form a space 37 having a shape of the objective lens 3.
When the pressing machine 24 is used for fabricating the objective lens 3, the upper mold 25 is first lifted as shown in FIG. 11A. A ball-shaped glass preform material 38 is supplied in the concave section 36 and is heated until the material reaches a specified temperature. As shown in FIG. 11B, the upper mold 25 is lowered to press the preform material 38 into the shape of the objective lens 3. Thereafter, as shown in FIG. 11C, the formed objective lens 3 is cooled. The upper mold 25 is lifted for allowing the objective lens 3 to be removed from the concave section 36.
As mentioned above, when the objective lens 3 is fabricated, the preform material 38 is placed in the concave section 36. At this time, the preform material 38 rolls on the concave section 36 for centering between the preform material 38 and the concave section 36. However, since the optical surface 19, namely the concave section 36, provides a large curvature radius, the preform material 38 does not efficiently roll on the concave section 36. The preform material 38 may be formed with incomplete centering between the preform material 38 and the concave section 36. In this case, as shown in FIG. 13, the preform material 38 flows into a gap formed among the cavities 29 and 34 and the dies 30 and 35, causing an over-packing phenomenon 39 to damage the cavities 29 and 34 and the dies 30 and 35. Alternatively, the preform material 38 is not filled completely in the space 37, causing an non-uniform section 40. There has been a possibility of generating asymmetrical components of aberration such as a spherical aberration, coma aberration, etc.